1. Field of the Invention
The invention relates to an illumination system and a projection apparatus.
2. Description of Related Art
With the advancement in display technology, various display apparatuses such as liquid crystal displays (LCDs), plasma displays, organic light emitting diode (OLED) displays, cathode ray tubes (CRTs), and projection apparatuses are now available for selection. Being capable of generating larger frames with more compact volume, projection apparatuses are irreplaceable in several usages, for example, conference briefing, home theater, classroom briefing when teaching or researching, and so on.
Currently, light valves applied in projection apparatuses are generally categorized into two categories; that is, digital micro-mirror device and liquid-crystal-on-silicon panel (LCOS) panel. Projection apparatuses adopting digital micro-mirror device display full color images using a color sequential method. In this type of projection apparatus, an illumination beam emitted by a white light bulb (e.g. a high pressure mercury lamp) passes through an ultraviolet and infrared filter to form the visible light (with a wavelength ranging from about 380 nm to 780 nm) and then filter different colors through a color wheel. Next, the illumination beam is converged on the digital micro-mirror device through a lens after the uniformization of a light integration rod. By switching on and off the digital micro-mirror device, the colors conforming to the image are reflected to a screen for imaging. Here, a light emitting efficiency of the entire optical system is about 25%. For example, a 180 W bulb is capable of emitting 8600 lm, but only 2150 lm is remained when the bulb is placed in the projection system. One of the devices having the lowest light efficiency is the color wheel. This is due to the fact that the color wheel adopts a sequential display and reflects the light of unneeded colors. For instance, when a red filter of the color wheel is moved into a light path, the red light passes through the red filter, but the green light and the blue light are reflected back to the high pressure mercury lamp as a heat source, thereby resulting in the loss in light energy.